This blog post is meant more for physicians who refer patients for EMG testing but please feel to read.
As a physician that performs electrodiagnostic evaluations (EMGs) I appreciate the fact that interpretation of the wave forms can have a certain degree of subjectivity to it. Many times I am asked by a referring physician to repeat an EMG test on a patient because the result they obtained do not fit with the clinical picture. If I am able to obtain a copy of the previous test I am sometimes struck by the diagnosis or diagnoses made and cannot understand how the physician came to the conclusion in the report.
Many people are impressed by the fancy graphs and tables that electromyography (EMG/NCS) reports typically generate. But, there is more to the study than a bunch of numbers and waveforms. Because most clinicians are not trained to interpret the raw data from an electrodiagnostic study, they are often intimidated by the reports and therefore accept the conclusion without question. As a Physiatrist, being trained to perform and evaluate electrodiagnostic studies works to my advantage. It allows me to evaluate both the quality of the study and to analyze the results. Unfortunately, not every report matches up with the data.
Factors affecting nerve conduction results
For example, did you know that the results of the nerve conduction study can be adversely affected by age, cooler environment, obesity, and swelling (edema), possibly leading to false positive results? While being cold may not seem like a problem in Florida, you must not forget that the patient may have been sitting in an air conditioned waiting room for a period of time prior to the study. While it is not common to document skin temperature in the report, an electromyographer should take this into consideration when analyzing the results. In addition, ‘senile polyneuropathy’ is common as you get older. This simply means that the nerves do not work as well as you age. Therefore, the physician should keep this in mind when evaluating the results of the nerve test. For example, the sural sensory response may be ‘absent’ in an 80 year old, but this does not necessarily reflect serious underlying pathology.
I have also seen people diagnosed with radiculopathies who do not meet the standard criteria for doing so. The criteria for diagnosing a radiculopathy via EMG is to have abnormal activity in at least 2 separate muscles controlled by the same nerve root, but different peripheral nerves. For example, the C5 nerve root contributes innervation to both the biceps and deltoid muscles. These muscles are controlled by different peripheral nerves. The biceps is controlled by the musculocutaneous nerve and the deltoid is controlled by the axillary nerve. If only the biceps shows abnormal activity, and the remainder of the test is normal, that particular patient does not meet the electrodiagnostic criteria for diagnosing a radiculopathy (pinched nerve). In addition, certain aspects of the EMG portion are more subjective than others. ‘Insertional Activity’ is relatively subjective, and I would be very cautious about diagnosing a radiculopathy based upon abnormal insertional activity alone. Muscle recruitment can be affected by patient’s discomfort, and therefore low recruitment is not always reflective of underlying pathology. While there are published standards for evaluating the EMG wave forms from a mathematical perspective, most physicians ‘eye ball’ the wave forms instead, which induces objectivity. In addition, it is possible to have pain and nerve root irritation with a normal EMG because electromyography does not test the pain fibers.
Above all, the results must make clinical sense. Sometimes too much ‘fishing’ is done with the test and results are presented that are not clinically relevant.
So, my take home message is to look beyond the intimidating graphs, tables, and wave forms to make sure the data not only correlates to the patient’s condition but also that the conclusions were properly reached. If you are unable to properly analyze the data yourself, seek the advice of a trained physician who can assist you in doing so.
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